1. Field of the Invention
The invention relates to a method of manufacturing a multi-layered ceramic circuit board containing layers of a reduced dielectric constant, for mounting elements such as a large scale integrated circuit, and particularly, to a method of manufacturing a multi-layered ceramic circuit board containing layers of a reduced dielectric constant of a type in which ceramic green sheets differing in their compositions are laminated in combination.
2. Description of the Related Art
In recently developed circuit boards for mounting large scale integrated circuit elements, higher speed signal transmission is preferred, and consequently, various types of circuit boards having a reduced dielectric constant have been developed, which are favorable for increasing the speed of signal transmission.
It is known in the art to use a hollow silica material contained in a ceramic circuit board to decrease the dielectric constant of the circuit board. JP 59-111345(A) discloses a low dielectric constant glass ceramic material used for circuit boards that contains a hollow ceramic material. The glass ceramic material is a composition containing a glass powder and a hollow ceramic powder such as hollow alumina powder as inorganic ingredients. The use of a hollow ceramic material, including hollow silica, as a material to reduce the dielectric constant of a ceramic circuit board is additionally disclosed in JP 61-83674(A), JP 62-206861(A), JP 2-83995(A), and U.S. Pat. No. 4,821,142. The above literature, however, does not refer to a hybrid circuit board in which non-conductive layers differing in composition, i.e., layers differing in dielectric constant, are laminated in combination.
A multi-layered circuit board that is made up of green sheets differing in composition and laminated in combination, is known. For example, JP 3-151690(A) discloses a multi-layered circuit board having a low dielectric constant and a high degree of strength, by hybrid-laminating green sheets of a low dielectric constant material and green sheets of a high strength material. No hollow silica materials are used in JP 3-151690(A).
U.S. Pat. No. 4,781,968 shows the formation of layers containing hollow glass microspheres and no microspheres on an alumina substrate. Shrinkage in a horizontal direction of laminated layers at the time of firing is not mentioned.
U.S. Pat. No. 4,620,264 discloses the use of hollow ceramic filler particles and refers to a hybrid structure as defined above. No reference is made to the shrinkage of green sheets during the firing when making a hybrid circuit board.
JP 60-254697(A) which corresponds to U.S. Pat. No. 4,761,325 discloses the use of glass ceramic material containing not less than 20% and less than 50% by weight of alumina, not less than 10% and less than 60% by weight of quartz glass, and not less than 20% and less than 40% by weight of glass or crystallized glass capable of being fired at a lower temperature than the melting point of copper, for the production of a multi-layered ceramic circuit board. However, neither the use of a hollow material nor a hybrid structure is referred to in JP 60-254697(A).
Although a green sheet containing a hollow ceramic material provides a low dielectric constant, such a green sheet causes the multi-layered circuit board produced therefrom to have a low degree of strength. By using a hybrid structure as well as a hollow ceramic material, multi-layered circuit boards were provided with a significant degree of strength and a low dielectric constant. Nevertheless, the prior multi-layered glass ceramic circuit boards having a hybrid structure and comprising low dielectric constant layers containing a hollow material are difficult to successfully manufacture, because such a hybrid multi-layered ceramic or glass ceramic circuit board suffers from delamination and/or cracks during the firing of laminated green sheets. This problem is considered in Japanese Patent Application No. 3-55188 filed Mar. 19, 1991, by way of adjusting the particle diameter of hollow microspheres contained in laminated green sheets for constituting layers of reduced dielectric constant so as to control the shrinkage difference in firing between the green sheets containing hollow microspheres and green sheets containing no hollow materials. (The content of Japanese Patent Application No. 3-55188 was published on Oct. 15, 1992, as JP 4-290492(A), after the filing date of Japanese Patent Application No. 4-41098, on which the priority of the present application is claimed.)